Development and Testing of an Aberration-Correction System for Photoelectron Microscopes and Other High Resolution Imaging Systems.

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The goal of this project is to achieve in electron optics what was done in light optics over two hundred years ago: that is, to develop the electron optical equivalent of the acromat. When completed, this will be the first practical device to simultaneously correct both spherical and chromatic aberrations in electron optics. The two key elements are an electron mirror and a beam separating system. The mirror has aberrations of opposite sign from those of conventional electron lenses and thus provides a means of canceling out aberrations. This grant renewal seeks funds to carry out the third phase of this project. The first phase covered the design and construction of an electron mirror, beam separator, and the electron lenses, and theoretical and experimental investigations of the mirror properties (G. F. Rempfer J Appl. Phys. 67, 6027-6040 (1990)). The second phase (just completed) produced the first successful demonstration of the correction of chromatic aberration by means of an electron mirror (G. F. Rempfer and M. S. Mauck, Optik (1992) 86, 3-8), and the completion of an advanced electron optical bench to carry out high precision testing of an aberration corrector. The goal of the proposed third phase is to utilize this unique resource, (the new electron optical bench, the only one in existence designed for branched systems), to demonstrate correction of spherical aberration, and simultaneous correction of chromatic and spherical aberration. When the conditions for correction have been established, resolution tests will be performed. Tests will be done first in the probe mode. The correction parameters will then be adjusted for cancellation of aberrations in the photoelectron microscope (PEM), and resolution tests will be performed in the PEM mode. The bench studies will lead to optimum positioning of the components of the optical system and locations of the feedthroughs, manipulators, and ports, so that a less bulky vacuum housing can be designed which wi ll serve the purpose of providing a workable prototype instrument. The new housing will free up the optical bench for continuing experiments with other branched systems. A promising byproduct of this research is the beam separator, which can serve to separate the incident and returning beams in a low energy electron microscope/mirror electron microscope (LEEM/MEM). In fact, the first LEEM and MEM images of biological specimens (cultured mammalian cells) were recently obtained with the beam separating system developed as part of this project (O H. Griffith et al., J Micros. 168, 249-258 (1991)). Because the branched system is common to both LEEM/MEM and corrected PEM, the new vacuum housing for the corrected-PEM can be designed to enable LEEM and MEM studies as well. The research planned for this proposed five-year NSF grant renewal will result in an aberration-corrected lens system (i.e. an electron optical acromat) and a modular instrument intended primarily for corrected PEM, but capable also of LEEM and MEM. The higher resolution PEM should produce sharp imagines of DNA and DNA-protein complexes having information (photoelectron contrast) not available in the present imaging methods. Other applications include receptor distributions on the surfaces of normal and malignant cells, and imaging membranes and cytoskeletal elements. A hallmark of this research collaboration between a physics and engineering team (at Portland State University in Portland) and a molecular biology group (at the University of Oregon, Eugene) is the rapid application in the biological sciences of the instrumental advances funded by this NSF grant.